A quadrature modulation scheme in which a carrier is modulated using an orthogonal baseband signal is widely used among mobile communication terminals such as cellular phones. In the quadrature modulation scheme, when a DC offset is included in an I/Q baseband signal of a quadrature modulator, an unnecessary carrier component called a “carrier leakage” is outputted. A large carrier leakage to signal ratio may cause deterioration of modulation accuracy or an increase of demodulation errors on a demodulator side.
In order to solve these problems, a method is proposed whereby a magnitude of a carrier leakage component with respect to a desired signal component is monitored through envelope detection and feedback control is performed so as to reduce the carrier leakage component.
However, when performing feedback control while monitoring the magnitude of the carrier leakage component through envelope detection, it is necessary to mount an envelope detector, a high-pass filter for DC cutting, a variable amplifier for level adjustment and an ADC that can detect a single-ended signal. This results in an increase in a circuit scale and a circuit area of the quadrature modulator, which may cause a problem that the size of the apparatus increases.